1. Field of the Invention
This invention relates in general to earth boring equipment and in particular to a stabilizer for centering a drill string in large diameter shafts.
2. Description of the Prior Art
In drilling of large diameter shafts, approximately 4 to 15 feet in diameter, a single drill bit with a diameter equal to that of the shaft is rotated by a string of drill pipe. The pipe, which is normally only 8 to 20 inches in diameter, is connected to the bit by a large flange, and thrust is applied to the bit by a stack of large do-nut shaped weights which surround the drill string and rest on the upper side of the flange. Because of the limberness of the drill pipe, the clearance between the weight stack and shaft wall, and the cutting action of these large bits, the bits have a tendency to wander from a straight path.
To restrict deviation and keep the bit on course a stabilizer is placed in the weight stack normally near the top. One type of stabilizer known to applicants has a cylindrical frame or hub which surrounds the drill pipe and is sandwiched between two weights. Extending from the central hub are radial arms with vertically oriented rollers mounted at their outer ends for rolling contact with the wall of the shaft.
There are several disadvantages with this type of stabilizer. The rollers have antifriction bearings which require an expensive structure for lubrication and protection from the drilling fluid. Also, the weight of the rollers necessitates a heavy supporting structure and makes them difficult to change. Furthermore, intermittent contact between the rapidly moving rollers and the shaft wall can promote a sloughing of the shaft wall.
Another type of stabilizer has a central structure similar to the roller type with radial arms supporting a continuous cylindrical ring. This arrangement affords simple construction and eliminates the expensive rollers. However, the rubbing of the ring against the wall of the shaft increases energy consumption and promotes wall sloughing and wear of the ring.
Another problem is that the opening in the rig floor is often too small in diameter to allow a stabilizer or bit to be moved through it. The rig floors are elevated a sufficient distance to allow the bit to be fully exposed in the space between the earth surface and the rig floor. However, there is often insufficient room for the bit and the stabilizer both to be exposed below the rig floor. Consequently, the stabilizer must be dismantled in order for the drill bit to be pulled above the earth surface. Dismantling a prior art stabilizer can be cumbersome and time consuming.